Heat treatment devices have been used to form diffusion layers or form silicon oxide or nitride films in the manufacture of electronic devices on semiconductor or glass substrates. These substrates are typically thin wafers made of silicon or other semiconductor material, and the description of the device hereinafter will be provided in reference to wafer substrates, it being understood that the apparatus is equally suitable for treating any thin glass or semiconductor sheets, and treatment of all of these materials are considered to be within the scope of this invention.
These devices provide the desired heat treatment by heating the wafers in a reactor or heating chamber while introducing inert or reactive gases into the chamber. These heating chambers are surrounded by heating elements enclosed within an insulated shell. In order to treat large numbers of wafers in a single heat treatment operation, it is conventional to support the wafers, one above the other in a parallel orientation, in a wafer boat. This combination is referred to hereinafter as a wafer stack.
The heat treatment apparatus and process must accomplish the heat treatment without damaging the wafers such as by causing slips or warping, for example. Therefore, severe temperature differences across the width of the substrates must be avoided. On the other hand, the heating process should be as brief as necessary to accomplish the desired treatment.
The transfer temperature of the wafers to and from the furnace or reactor is from 500.degree. to 600.degree. C. The heating cycle is initiated after the wafer stack, at the transfer temperature, is positioned in the heating chamber, and ambient gases have been replaced by suitable protective gases. The usual wafer spacing in the wafer stack is approximately 4 mm per wafer. Heat radiating from the heating elements surrounding the heating chamber impinge primarily on the outer edges of the wafers. Increasing the heating and cooling rates causes an increase in temperature differences between the outer edge and center of the wafers, and an excessive increase in heating and cooling rates causes thermal distortions, leading to warping, crystal defects and slips in the wafers. The maximum heating rate in such configurations is limited to 10.degree.-12.degree. C./min, and the maximum cooling rate is about 5.degree. C./min. This extends the time of each thermal treatment cycle and severely limits the production capacity or throughput of each thermal treatment device.
Japanese patent application publication Hei 4-184923 to Nishimura et al (Jul. 1, 1992) describes a heat treatment apparatus designed to reduce the heating time. In this apparatus, the wafers are supported in a circular jig having a heat capacity graduated to be maximum at the periphery of the wafers. The jigs comprise ring-shaped trays which hold the wafers around their peripheries, the thickness of the heat capacity of the tray being constant or increasing from the inside to the outside. The trays can be formed from several materials in which the specific heat is greater on the outside than the inside. The Nishimura et al system greatly increases the heat capacity of the components in the heating chamber, requiring the provision of greater heat energy for the heating phase and greater heat removal during the cooling phase of the cycle. The minimum times of both the heating and cooling phases are extended by these high heat capacity components.